Across-sectional study is one in which different characteristics of a given individual are assessed at the same time. the behavior of a group is assessed by different researchers. the same people are retested over a period of years. different age groups are tested at the same time.

When there is a cross- sectional study, the data are then collected on the whole study population at a single point in time. There is the process of testing different age groups at the same time. The purpose of this is to examine the relationship between diseases or other health related states, and other variables of interest. For a cross-sectional study’s findings to be valid there should be a representation of a whole population. Take for instance, a study of the prevalence of diabetes among women aged 40-60 years in Town A should be composed of a random sample of all women aged 40-60 years in that town. Attempts should be made to include hard to reach groups, such as people in institutions or the homeless for this study to be said that there is proper representation. 

nasa 3

answer:

d. because dr. laguardia selected a widely used measure, his study generalizes to has strong external validity. external validity has limited external validity. other applications of this measure.

the widely used measure that he applied to his experiment was the benton facial recognition test. it employed on to football players and as result, only players who suffered brain concussions did not pas this test. and although it has an external validity, it is limited because it isn't general to apply to other population.